In many RF communication applications, such as applications according to the ZigBee standard, use is made of a direct up-conversion transmitter architecture wherein a local oscillator is operating at twice the output frequency (operational transmitting frequency). This type of transmitter architecture is susceptible to oscillator injection pulling (or oscillator re-modulation), which is an unwanted effect in RF design. The article by B. Razavi, ‘A Study of Injection Locking and Pulling in Oscillators’, IEEE Journal of Solid-State Circuits, vol. 39, no. 9, September 2004, pp. 1415-1424, describes this phenomenon.
A possible solution to mitigate the effects of injection pulling would be to shift operation of the local oscillator to four times or two thirds of the output frequency, but this will result in increased power consumption and/or increased silicon area (if operating at two thirds of the output frequency).